The present invention relates to a high-frequency (for example, 600 MHz) crystal oscillator circuit, and in particular to a crystal oscillator circuit wherein design is simplified and changes in frequency are prevented.
Since the frequency of a crystal oscillator circuit is very stable, it is employed as a frequency source in a variety of electronic equipment. Recently, crystal oscillator circuits having a high oscillation frequency are in demand due to the need for development of optical communications systems. One of these crystal oscillator circuits is the high-speed ECL (Emitter Coupling Logic) employed as an oscillation amplifier.
FIG. 6 through FIG. 8 explain a conventional example of a crystal oscillator circuit. FIG. 6 shows the crystal oscillator circuit, FIG. 7 the internal circuit of the ECL, and FIG. 8 a simplified crystal oscillator circuit.
As shown in FIG. 6, the crystal oscillator circuit comprises a resonance circuit 1 shown within the dotted line and an oscillation amplifier 2. The resonance circuit 1 comprises a crystal element 3 and dividing capacitors 4a and 4b. The crystal element 3 is of the AT cut type, and functions as an inductor component. The dividing capacitors 4a and 4b are connected at either end of the crystal element 3, forming the input and output side of the oscillation amplifier 2, with each being connected to ground.
As explained above, the oscillation amplifier 2 comprises the ECL which integrates a differential amplifier, with two inputs (Ai, Bi), and two outputs (Co, Do) having opposite phases. For example, as shown in the interior of the ECL within the dashed line in FIG. 7, the emitters of first and second transistors TR1 and TR2 are connected to a common ground. Moreover, these have input ends (Ai, Bi) which apply opposite phase signals to each base, with each collector at a power supply Vcc.
Furthermore, as shown in FIG. 7, third and fourth transistors TR3 and TR4 are connected to the first and second transistors TR1 and TR2, and have output terminals (Co, Do) which obtain opposite phase signals from the emitters. Normally, pull-down resistors 9a and 9b for controlling emitter current are connected to each output terminal (Co, Do) as external loads. Normally, the pull-down resistors 9a and 9b have large resistance values in a range of, for example 150 to 200Ω, to prevent heating due to excessive direct current and to stabilize operation.
In this ECL, regarding the input terminals (Ai, Bi), a base voltage Vbb is applied as a bias voltage from the input terminal Ai to the input terminal Bi via a high-frequency blocking resistor 7 as shown in FIG. 6 and FIG. 8. Moreover the input terminals are connected to ground via a bypass capacitor 8. The base voltage is generated from the power supply Vcc by a circuit not shown in the drawings. However a bypass capacitor is unnecessary when driven with two power supplies.
Both ends of the crystal element 3, as shown in FIG. 6, are connected between the mutually opposite phase input and output (between Bi and Co) of the ECL employed as the oscillator. A buffer amplifier 5 employing a similar ECL to that for oscillation, is connected to the next stage of the oscillation amplifier. FIG. 8 shows the system simplified to include only the oscillator, with the pull-down resistor 9a between one input-output pair (Bi, Co), affecting the oscillator system.
In such a crystal oscillator circuit, the oscillation amplifier (ECL) 2 feedback amplifies the resonant frequency of the resonance circuit 1 connected between one input-output pair (Bi, Co) and maintains a square wave oscillation. The use of differential amplification allows input-output of a phase opposite to one input-output pair (Bi, Co) to be obtained at the other input-output pair (Ai, Do). Two outputs of the opposite phase, with the pull-down resistor 9a, 9b as load, are amplified by the buffer amplifier 5 (ECL) and a binary oscillation is output. The oscillation frequency almost matches the resonant frequency, and is determined precisely by the capacity of the load on the side of the circuit as seen from the crystal element 3.
However, in the conventional crystal oscillator circuit (see FIG. 6) of the aforementioned configuration, the effective capacity of the dividing resistor 4b changes due to the pull-down resistor 9a, resulting in difficulties in design. That is to say, since the pull-down resistor 9a is connected in parallel with the dividing capacitor 4b, the effective capacity of the dividing capacitor 4b becomes that of an equivalent series capacitance with the pull-down resistor 9a. Therefore the resonant frequency of the resonance circuit 1 is affected by the pull-down resistor 9a, resulting in difficulties in design. As a result, for example fluctuations occur in the oscillation frequency and oscillation gain.
Moreover, the equivalent series resistance with the pull-down resistor 9a becomes a load on the resonance circuit, and the sharpness of the resonance peak, that is to say, the Q value, is reduced, and for example, phase noise characteristics deteriorate. Furthermore, the equivalent series capacitance is less than that of the dividing capacitor 4b. Therefore, if for example, the dividing capacitor 4b of the output side comprises a voltage-controlled oscillator as a variable capacity diode, there is also a problem in that the range of variation of the oscillation frequency is reduced.
Furthermore, since the high-frequency blocking resistor 7 is connected in parallel with the input dividing capacitor 4a, the effective capacity of the dividing resistor 4b changes, therein resulting in difficulties in design as aforementioned. However, since the high-frequency blocking resistor 7 has a high resistance value, its effect is less than that of the pull-down resistor 9a of the output side. Even so, this problem cannot be ignored.